Etching process to selectively remove copper plating seed layer

ABSTRACT

Write head coils for magnetic disk systems are commonly formed through electroplating onto a seed layer in the presence of a photoresist mask. It is then necessary to remove the seed layer everywhere except under the coil itself. The present invention achieves this through etching in a solution of ammonium persulfate to which has been added the complexing agent 1,4,8,11 tetraazundecane. This suppresses the reduction of Cu ++  to Cu, thereby increasing the dissolution rate of copper while decreasing that of nickel-iron. Two ways of implementing this are described—adding the complexing agent directly to the ammonium persulfate and introducing the 1,4,8,11 tetraazundecane through a dipping process that precedes conventional etching in the ammonium persulfate.

FIELD OF THE INVENTION

The invention relates to the general field of metal etching,particularly to etching copper in the presence of nickel-iron, and withparticular application to the manufacture of write heads for magneticdisk systems.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, we show, in schematic representation, across-sectional view of a write head for a magnetic disk system. Themagnetic field needed to perform the write operation is generated byflat coil 16 made up of a number of turns (typically between about 8 and18), where 13 is an example of one side of a single turn. Surroundingthe flat coil is magnetic material comprising upper and lower polepieces 12 and 11 respectively. These pole pieces are joined at one end(on the left in this figure) and are separated by small gap 14 at theother end. The magnetic field that is generated by flat coil 16 ends upbeing concentrated at gap 14. It is sufficiently powerful that thefringing field that extends outwards away from gap 14 is capable ofmagnetizing the magnetic storage medium over whose surface 15 the head‘flies’. The distance between gap 14 and surface 15 is typically betweenabout 10 and 50 nm.

Clearly, flat coil 16 cannot be in direct contact with lower pole piece11 so there is always a layer of insulating material (not shown inFIG. 1) between the coil and the lower pole piece. Since the coil isbuilt up using electroplating, it is necessary to provide a conductivelayer to act as a seed for the initiation of plating. Once the coil hasbeen grown over this seed layer, the latter must be removed (exceptwhere it is directly under the coil) to avoid shorting out the coil.

Two general approaches to the problem of removing the seed layer arepracticed in the prior art. The first is sputter etching. This approachis limited by the tendency for some of the sputter etched material to bere-deposited, so some shorting out of the coil remains a possibility.The second approach is to use chemical etching. In principle thisovercomes the difficulties associated with sputter etching but thismethod, too, has associated difficulties. Unfortunately, etchants usedto remove copper (notably ammonium persulfate) also attack permalloy (inparticular, the nickel component of the permalloy), introducing thepossibility of damaging the lower pole piece 11 during seed layerremoval. This can happen because there is an ongoing possibility thatsome of the copper seed layer will be in direct contact with the lowerpole piece. Such contact with the copper seed layer is possible becauseupper pole 12 must contact lower pole 11 at one end so it is convenientto initially plate it on the exposed area of lower pole 11. It is alsonecessary to electroform a copper stud near the device in order to makeelectrical wiring connection after the upper pole has been completed.Since the seed layer covers the whole wafer, there is direct contact ofthe copper seed layer on top of upper pole 12 which should be removedafter stud plating. The present invention provides a solution to thisproblem.

A routine search of the prior art was performed but no processes similarto the exact process of the present invention were found. Severalreferences of interest were, however, encountered. For example, U.S.Pat. No. 5,304,284 (Jagannathan et al.) shows a process for etchingcopper in the presence of more reactive metals. The method depends onthe use of non-aqueous solutions and is based on creating a suitablebalance between solutes of different oxidation potentials. Oneembodiment uses alkenes and alkynes as complexing agents.

In U.S. Pat. No. 5,800,726, Cotte et al. address the inverse problem tothat solved by the present invention. i.e. it is the copper that is tobe protected while other metals are being etched. Heim et al. (U.S. Pat.No. 5,935,644) show a pole and coil plating process while in U.S. Pat.No. 5,639,509 Schemmel teaches a process for forming a magnetic datatransducer.

SUMMARY OF THE INVENTION

It has been an object of the present invention to provide a process formanufacturing a write coil for use in the write head of a magnetic disksystem.

Another object of the invention has been to use electroplating on a seedlayer and to subsequently remove said seed layer without affecting othermaterials that are close by, notably nickel-iron.

A further object of the invention has been that removal of the seedlayer be accomplished by means of wet etching.

These objects have been achieved by removing the seed layer throughetching in a solution of ammonium persulfate to which has been added thecomplexing agent 1,4,8,11 tetraazundecane. This suppresses the reductionof Cu⁺⁺ to Cu, thereby increasing the dissolution rate of copper whiledecreasing that of nickel-iron. Two ways of implementing this aredescribed—adding the complexing agent directly to the ammoniumpersulfate and introducing the 1,4,8,11 tetraazundecane through adipping process that precedes conventional etching in the ammoniumpersulfate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a write head for a magnetic disksystem.

FIG. 2 is the starting point for the process of the present inventionshowing a photoresist pattern on a copper seed layer.

FIG. 3 shows the structure of FIG. 2 after formation of the coil throughelectroplating.

FIG. 4 shows the end product of the process of the present invention.

FIG. 5 shows the molecular structure of complexing agent 1,4,8,11tetraazundecane.

FIGS. 6 and 7 are polarization curves for copper and nickel-ironrespectively plotted in ammonium persulfate solution with varyingamounts of added 1,4,8,11 tetraazundecane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

We begin a description of the process of the present invention at thepoint where a write coil (such as 16 in FIG. 1) is about to be formed.Referring now to FIG. 2, we show (in schematic cross-section) part of abody of nickel-iron 21. Most commonly, said body would be the upperportion of a lower pole piece (such as 11 in FIG. 1) but the inventionis not limited to such hardware, being applicable to any nickel-ironbody.

Insulating layer 22 is then deposited on the upper surface of body 21.Preferably, insulating layer 22 is photoresist that has been fullydeveloped and then baked at a temperature between about 200 and 300° C.for between about 3 and 6 hours in nitrogen. The thickness of layer 22is between about 0.5 and 4 microns. Note that the use of photoresist forlayer 22 is not essential for the present invention to work and thatother insulating materials such as aluminum oxide or magnesium oxidecould also have been used.

With layer 22 in place, copper seed layer 23 is then laid down overinsulating layer 22. Our preferred method for depositing 23 has beensputtering but other deposition methods such as evaporation or chemicalvapor deposition (CVD) could also have been used without departing fromthe spirit of the invention. Layer 23 is between about 300 and 1,000Angstroms thick.

Next, photoresist layer 24 is laid down on seed layer 23. Layer 24 mustbe thicker than the final thickness intended for the still-to-be-formedflat coil. Layer 24 is exposed to a suitable pattern and then developedso that it takes on the negative (inverse) image of the flat coil. Thisimplies that the underlying seed layer is exposed in areas that definethe coil and covered everywhere else. At this stage of the process thestructure has the appearance seen in FIG. 2.

The step of forming the coil is illustrated in FIG. 3. By means ofelectroplating, the copper thickness is increased in all the exposedareas. The additional amount of copper that is added in this manner isbetween about 1 and 5 microns. Then, once the photoresist has beenstripped away, the structure has the appearance shown in FIG. 3. Alsoillustrated in FIG. 3 is area 35 where insulating layer 22 has beenpenetrated by seed layer 35, bringing it into contact with nickel-ironbody 21. As already discussed earlier, such breaching of layer 22 cancome about because of connection between upper pole 12 and lower pole11, as well as because of the stud plating process.

Once the stage illustrated in FIG. 3 has been reached, seed layer 23needs to be removed everywhere except under the coil. This has to beachieved without affecting nickel-iron body 21 and the sub-process foraccomplishing this constitutes the heart of the present invention. Wenow disclose two separate embodiments of said sub-process:

First embodiment

The structure, as shown in FIG. 3, is immersed in an aqueous etchingsolution of ammonium persulfate and 1,4,8,11 tetraazundecane for betweenabout 4 and 50 seconds at a temperature between about 10 and 25° C. Theconcentration of the ammonium persulfate is between about 40 and 50gms./liter while that of the 1,4,8,11 tetraazundecane is between about01 and 1 gms./liter. Under these conditions the copper seed layer isfully removed while the nickel-iron is not affected, giving thestructure the appearance illustrated in FIG. 4. Note that it isnecessary that the pH of this etching solution be between about 12 and14. If needed, this level of pH can be brought about through theaddition of a suitable quantity of ammonium hydroxide.

Second embodiment

An alternative approach to that outlined above is to use a two stageprocess. First, the structure is dipped in an aqueous solution of1,4,8,11 tetraazundecane, having a concentration between about 0.3 and1.5 gms./liter, for between about 0.5 and 5 minutes at a temperaturebetween about 17 and 25° C., and then immediately transferred to anaqueous etching solution of ammonium persulfate at a concentration ofbetween about 40and 50 gms./liter, where it is immersed for betweenabout 5 and 50 seconds at a temperature between about 10 and 25° C.Under these conditions the copper seed layer is fully removed while thenickel-iron is not affected. It is necessary that the pH of the etchingsolution be between about 12 and 14 while that of the dip should bebetween about 6 and 14. If needed, these levels of pH can be broughtabout through the addition of a suitable quantity of ammonium hydroxide.If these conditions are met, the copper seed layer is fully removedwhile the nickel-iron is not affected, giving the structure theappearance illustrated in FIG. 4.

By way of explaining why the process of the present invention iseffective in suppressing the dissolution of nickel-iron relative tocopper we note that the dissolution of copper by ammonium persulfate isan oxidation reaction of the form

Cu⇄Cu⁺⁺

so that any action that drives this reaction more to the right hand sidewill result in an increase in the dissolution rate of copper withoutaffecting the dissolution rates of other materials present in the samesolution. The action of the 1,4,8,11 tetraazundecane, whose molecularstructure is illustrated in FIG. 5, is to serve as a complexing agentthat keeps the Cu⁺⁺ ion in solution while at the same time making itunavailable for reduction, thereby keeping the active concentration of[Cu⁺⁺] low.

Experimental confirmation of the above mechanism was obtained byplotting separate polarization curves for both copper and nickel-iron inammonium persulfate, with and without the addition of the 1,4,8,11tetraazundecane complexing agent. A polarization curve measures thecorrosion exchange current and the corrosion potential by plottingIg(I/A) vs. voltage applied between a metal electrode (in this casecopper or nickel-iron) and a standard reference electrode (in this casea standard calomel electrode).

The results are shown in FIGS. 6 and 7 which are for copper andnickel-iron respectively. In FIG. 6, curve 61 is for pure ammoniumpersulfate while curves 62 and 63 are for ammonium persulfate with 0.3g/liter and 0.6 g/liter, respectively, of added 1,4,8,11tetraazundecane. In FIG. 7, curve 71 is for pure ammonium persulfatewhile curves 72 and 73 are for ammonium persulfate with 0.3 g/liter and0.6 g/liter, respectively, of added 1,4,8,11 tetraazundecane. Theseresults show that the addition of 1,4,8,11 tetraazundecane to theammonium persulfate solution makes the copper potential more cathodic(moves it to the left) while it has the opposite effect on thenickel-iron.

Finally, we note that although the process of the present invention hasbeen disclosed in terms of a specific complexing agent (1,4,8,11tetraazundecane), it would be obvious to one skilled in the art toachieve similar results to those of the present invention by use ofother, similar, complexing agents such as 1,5,9,13 tetraazatridecane ortriethylene tetramine. So, while the invention has been particularlyshown and described with reference to the preferred embodiments thereof,it will be understood by those skilled in the art that various changesin form and details may be made without departing from the spirit andscope of the invention.

What is claimed is:
 1. A process for etching copper in the presence ofnickel-iron, comprising: providing a body of nickel iron and a layer ofcopper in the vicinity of the nickel-iron such that there is contactbetween the copper and the nickel-iron; and immersing the layer ofcopper and the body of nickel-iron in an aqueous etching solution ofammonium persulfate, at a first concentration, to which has been added1,4,8,11 tetraazundecane at a second concentration, whereby said copperlayer is removed and the nickel-iron is unaffected.
 2. The process ofclaim 1 wherein the concentration of ammonium persulfate is betweenabout 40 and 50 gms./liter.
 3. The process of claim 1 wherein theconcentration of 1,4,8,11 tetraazundecane is between about 0.1 and 1gms./liter.
 4. The process of claim 1 wherein said aqueous etchingsolution has a pH between about 12 and
 14. 5. A process for pre-wettingcopper prior to etching in the presence of nickel-iron, comprising:providing a body of nickel iron and a layer of copper in the vicinity ofthe nickel-iron such that there is contact between the copper and thenickel-iron; dipping the body having nickel-iron and copper in anaqueous solution of 1,4,8,11 tetraazundecane; removing the body havingnickel-iron and copper from said solution of 1,4,8,11 tetraazundecane;and immediately immersing the body having copper and nickel-iron in anaqueous etching solution of ammonium persulfate whereby said copperlayer is removed and the nickel-iron is unaffected.
 6. The process ofclaim 5 wherein the ammonium persulfate has a concentration in theetching solution of between about 40 and 50 gms./liter.
 7. The processof claim 5 wherein the 1,4,8,11 tetraazundecane has a concentration inthe aqueous solution of between about 0.3 and 1.5 gms./liter.
 8. Theprocess of claim 5 wherein said aqueous etching solution has a pHbetween about 12 and 14 and the aqueous solution of 1,4,8,11tetraazundecane has a pH between about 6 and 14.